Fiber optic personnel safety systems and methods of using the same

ABSTRACT

A personnel monitoring system. The personnel monitoring system includes a host node having an optical source for generating optical signals, and an optical receiver. The personnel monitoring system also includes a plurality of fiber optic sensors for converting at least one of vibrational and acoustical energy to optical intensity information, each of the fiber optic sensors having: (1) at least one length of optical fiber configured to sense at least one of vibrational and acoustical energy; (2) a reflector at an end of the at least one length of optical fiber; and (3) a field node for receiving optical signals from the host node, the field node transmitting optical signals along the at least one length of optical fiber, receiving optical signals back from the at least one length of optical fiber, and transmitting optical signals to the optical receiver of the host node.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/607,716, filed Jan. 28, 2015, which is a continuation ofU.S. patent application Ser. No. 13/321,856, filed Oct. 10, 2012, whichis the U.S. national phase of International Application No. PCT/US2011/025206 filed on Feb. 17, 2011, which claims priority of U.S.Provisional Application No. 61/338,466 filed on Feb. 18, 2010, and U.S.Provisional Application No. 61/367,634 filed on Jul. 26, 2010.

TECHNICAL FIELD

This invention relates generally to tracking of personnel such as withina mine or aboard a large marine vessel and, more particularly, toimproved systems and methods for tracking of personnel using fiberoptics.

BACKGROUND OF THE INVENTION

The mining industry has always been beset with disasters. Such disastersmay be caused by explosions or cave-ins, and have resulted in seriousinjury and/or death to workers. Many of these injuries to miners (anddeaths) could have been prevented had adequate systems been in place fortracking the locations of miners within a mine, and had adequatecommunications been in place between surface personnel and miners afterelectrical power in the mine had been severed. In 2006, the UnitedStates Congress identified a need for improved mining safety equipment,including the ability to track personnel at all times and to providebi-directional communications following a disaster without the need forlocal (in-mine) electrical power.

Systems have been proposed and developed to address these concerns, butsuch systems suffer from significant drawbacks. For the neededbi-directional communications, relayed 2-way radios have been employed;however, such radios typically utilize a number of fixed stations, eachrequiring electrical power. Further, communication ranges of such 2-wayradios tend to be too short for many mining applications. Still further,radio frequency (RF) communications are poor in many miningenvironments. For personnel tracking, coaxial cable systems have beenproposed; however, they are known to have leakage issues that inhibittheir effectiveness. Radio frequency identification (RFID) systems havealso been proposed; however, such systems are typically arranged in adaisy chain configuration with multiple fixed stations, each requiringlocal electrical power.

Thus, a need exists for, and it would be desirable to provide improvedsystems for, monitoring and/or tracking of personnel.

BRIEF SUMMARY OF THE INVENTION

To meet this and other needs, and in view of its purposes, the presentinvention provides a personnel monitoring system. The personnelmonitoring system includes a host node including an optical source forgenerating optical signals, and an optical receiver. The opticaldetection system also includes a plurality of fiber optic sensors forconverting vibrational energy to optical intensity information, each ofthe fiber optic sensors including: (1) at least one length of opticalfiber configured to sense vibrational energy; (2) a reflector at an endof the at least one length of optical fiber; and (3) a field node forreceiving optical signals from the host node, the field nodetransmitting optical signals along the at least one length of opticalfiber, the field node receiving optical signals back from the at leastone length of optical fiber, and the field node transmitting opticalsignals to the optical receiver of the host node.

According to another exemplary embodiment of the present invention, amethod of operating a personnel monitoring system is provided. Themethod includes the steps of: (a) storing a plurality of predeterminedcharacteristics of events to be monitored using an optical detectionsystem in memory; (b) comparing a detected characteristic obtained fromthe optical detection system to the plurality of predeterminedcharacteristics stored in memory; and (c) determining if there is anacceptable level of matching between the detected characteristic and atleast one of the plurality of predetermined characteristics stored inmemory.

According to an exemplary embodiment of the present invention, anotherpersonnel monitoring system is provided. The personnel monitoring systemincludes a host node having an optical source for generating opticalsignals, and an optical receiver. The personnel monitoring system alsoincludes a fiber optic sensing cable having at least one sensing zone,the at least one sensing zone being bound by a pair of Fiber BraggGratings of the fiber optic sensing cable.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings. It is emphasizedthat, according to common practice, the various features of the drawingsare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawings are the following figures:

FIG. 1A is a block diagram illustrating an optical detection system inaccordance with an exemplary embodiment of the present invention;

FIG. 1B is a block diagram illustrating the optical detection system ofFIG. 1A used in connection with a mine in accordance with an exemplaryembodiment of the present invention;

FIG. 1C is a block diagram illustrating the optical detection system ofFIG. 1A used in connection with a marine vessel in accordance with anexemplary embodiment of the present invention;

FIG. 1D is a perspective view illustrating the optical detection systemof FIG. 1A used in connection with vehicle detection in accordance withan exemplary embodiment of the present invention;

FIG. 2 is a block diagram of a host node of an optical detection systemin accordance with an exemplary embodiment of the present invention;

FIG. 3 is a block diagram of a first field node of an optical detectionsystem in accordance with an exemplary embodiment of the presentinvention;

FIG. 4 is a block diagram of an intermediate field node of an opticaldetection system in accordance with an exemplary embodiment of thepresent invention;

FIG. 5 is a block diagram of a final field node of an optical detectionsystem in accordance with an exemplary embodiment of the presentinvention;

FIG. 6 is an external illustration of an intermediate field node of anoptical detection system in accordance with an exemplary embodiment ofthe present invention;

FIG. 7 is a cross-sectional view of a fiber optic cable in accordancewith an exemplary embodiment of the present invention;

FIG. 8 is a block diagram illustrating another optical detection systemused in connection with a mine in accordance with an exemplaryembodiment of the present invention;

FIG. 9 is a view of a length of cable in a personnel safety system inaccordance with an exemplary embodiment of the present invention;

FIG. 10 is a flow diagram illustrating a method of operating an opticaldetection system in accordance with an exemplary embodiment of thepresent invention; and

FIG. 11 is a flow diagram illustrating another method of operating anoptical detection system in connection with a mine in accordance with anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In order to enable detection and communication in connection with apersonnel safety system (e.g., a mine safety system, or other personnelsafety system), it is desirable to have a high fidelity electronicrepresentation of an event (e.g., acoustic vibration, mechanicalvibration, etc.). According to certain exemplary embodiments of thepresent invention, an optical detection system for personnel safety isprovided which utilizes interferometers with high linearity and dynamicrange (e.g., certain linearized Sagnac interferometers). The opticaldetection systems may also include a low noise, low distortion, opticalreceiver.

In certain more specific exemplary embodiments of the present invention,optical detection systems for personnel safety are provided whichutilize an integrated sensor array (e.g., including a sensing cabledivided into sensing zones which may be arranged to include a series oflinearized Sagnac interferometers) for monitoring systems and locations.Such optical detection systems may include a host node having aninterrogation sub-system and a signal processor.

In other exemplary embodiments of the present invention, a contiguousarray of Fiber Bragg Grating (i.e., FBG) bounded interferometers (e.g.,Fabry-Perot interferometers) interrogated by a Time DivisionMultiplexing (i.e., TDM) interferometric demodulator is provided.

Through various exemplary embodiments of the present invention, passivefiber optic personnel safety systems are provided. Use of passive fiberoptic sensing allows for the omission of electrical power for operationof the sub-systems residing within the mine or other area to bemonitored.

In yet another embodiment, bi-directional communications are included inthe fiber optic detection system (e.g., in a mine, vessel, or otherlocation). In a mine application, passive uplink communications fromwithin the mine to a mine office (e.g., a control room) are enabled byhigh sensitivity detection of voice at particular field nodes by use offiber optic microphones. The microphones are parts of the sensing zonesused for tracking individuals in the mine. Within the sensors, theacoustic voice information is converted to optical phase signals, andthen into optical intensity signals. At the host node, these opticalintensity signals are converted into electric, and then acoustic,signals for audible detection of the in-mine voices.

Further, to complete the bi-directional communications, optical downlinkcommunication from the mining office to the mine is provided. This isaccomplished by conversion of voice acoustics into electrical signalsvia a microphone and amplifier. The electrical signal is then imposedupon the output of a laser via Pulse Width Modulation (PWM) or othersuitable mechanisms. The resultant optical signal is transmitted alongan optical fiber of a fiber optic cable into the mine. At particularlocations in the mine, fiber optic earphones are provided for receptionof the voice signals generated in the mine office. The optical signal isreceived at an optical earplug where a photodetector causes anoptical-to-electrical conversion. Part of the electrical energy isrectified, filtered, and used as bias energy for a small electricalcircuit that converts the electrical signal to a baseband (demodulated)acoustic signal output through a miniature loudspeaker within theearphone for audible detection by a miner.

Referring now to the drawings, in which like reference numbers refer tolike elements throughout the various figures that comprise the drawing,FIG. 1A illustrates an optical detection system 10. Optical detectionsystem 10 includes a plurality of fiber optic cables (i.e., opticalsensing cables) 400 a, 400 b, 400 c . . . 400 n (which may be termedtransducers) configured into separate sensing zones 450, 455, 460 . . ., 499. Optical detection system 10 also includes a host node 100 and aplurality of field nodes. The field nodes include a first field node300, intermediate field nodes 500 a, 500 b, etc., and a final field node600. Optical detection system 10 also includes a lead cable 200 (e.g., alead cable for telemetry of probe and return signals from each of thezones, a length of such lead cable being application dependent, with anexemplary lead cable being on the order of meters to kilometers inlength) which runs between host node 100 and first field node 300, leadcable 200 preferably being acoustically and vibrationally insensitive.In the example shown in FIG. 1A, optical detection system 10 includes asingle host node 100, and a single first field node 300. Depending onthe exact configuration of the system (e.g., the number of sensingzones, the length of the cables covering each of the sensing zones,etc.), there may be a plurality of host nodes, first field nodes, etc.,as is desired in the given application.

An exemplary operation of the configuration illustrated in FIG. 1A maybe summarized as follows. Host node 100 (which works in conjunction witha signal processor 700) generates optical signals and transmits thesignals along lead cable 200 to first field node 300 (e.g., where theelements and configuration of optical detection system 10, includinglead cable 200, are selected to minimize the lead cable sensitivity tovibration). As will be detailed below, part of the optical signals fromhost node 100 (intended for use in monitoring sensing zone 450) aretransmitted through first field node 300, along optical sensing cable400 a, are reflected back after reaching intermediate field node 500 a,the reflected signals returning along optical sensing cable 400 a, andthe signals ultimately returning to host node 100 and signal processor700 for processing. Another part of the optical signals from host node100 (intended for use in monitoring sensing zone 455) is transmittedthrough first field node 300, along optical sensing cable 400 a, throughintermediate field node 500 a, along optical sensing cable 400 b, isreflected back after reaching intermediate field node 500 b, thereflected signals returning along optical sensing cables 400 b, 400 a,and the signals ultimately returning to host node 100 and signalprocessor 700 for processing. A similar process occurs for eachsubsequent sensing zone. As is clear in FIG. 1A, subsequent sensingzones (as indicated by zones 460 . . . 499) are contemplated, with thefinal sensing zone terminating with final field node 600.

The system described above with respect to FIG. 1A allows for sensing ofvarious acoustic and mechanical vibration events. FIG. 1A alsoillustrates beacons 775. In an exemplary personnel safety system (e.g.,a mine safety system), each person to be monitored (e.g., each miner)carries one of beacons 775. Each beacon 775 emits unique acousticvibrations (e.g., acoustic vibrations at a unique and predeterminedfrequency) that are previously known to the system and are detected byoptical sensing cables 400 a, 400 b, etc. Prior to entering a monitoredarea, each individual is associated with a particular beacon 775 by anysuitable mechanism.

FIG. 1A also illustrates a microphone 800 and one or morespeakers/headphones 900. As will be explained below in connection withFIG. 1B, these elements, in combination with like elements (e.g., afiber optic microphone 510 a and a fiber optic earplug 510 b in FIG. 6)in the mine (or other location to be monitored), form the basis for afiber optic bi-directional communications system requiring no electricalpower locally along the full sensing array.

FIG. 1B illustrates optical detection system 10 of FIG. 1A used in amine monitoring application. In FIG. 1B, host node 100 and signalprocessor 700 are housed in a control room 160 or other desirableenvironment (e.g., a remote, stable environment). FIG. 1B illustratesoptical detection system 10 configured to sense disturbances (e.g.,presence of miners' beacons, voices, etc.) within a mine 165 (e.g.,below ground level 165 a and above mine floor 165 b), where each sensingzone 450, 455, 460 . . . 499, corresponds to a given area of mine 165.

As provided above, each beacon 775 emits unique acoustic vibrations(e.g., acoustic vibrations at one or more unique and predeterminedfrequencies) that are detected by optical sensing cables 400 a, 400 b,etc. As will be understood by those skilled in the art, control roomelectronics 160 a transmit optical energy to the optical sensing cables,and optical energy is returned from each of the sensing zones 450, 455,460 . . . 499 along lead cable 200 (where the optical energy is changedby the received acoustic vibrations). Control room electronics 160 a candistinguish one beacon 775 from another (because of the unique signatureof the optical signals based on the frequencies or temporalcharacteristics of the signals emitted by a given beacon 775), and assuch, personnel wearing beacon 775 (e.g., a miner wearing a beacon) maybe tracked as they move from one sensing zone 450, 455, 460 . . . 499 toanother sensing zone 450, 455, 460 . . . 499.

As provided above, microphone 800 in control room 160 receives humanvoices (an acoustic signal) in control room 160, and together with otherelements of host node 100 converts the acoustic signal into anelectrical signal, and the electrical signal is converted to an opticalsignal transmitted along the fiber optic array (including lead cable 200and optical sensing cables 400 a, 400 b, etc.). This may beaccomplished, for example, by Pulse Width Modulation of the injectioncurrent to a laser based upon the electrical signal from the convertedacoustic voices. In an exemplary embodiment, Pulse Width Modulation isapplied at a frequency on the order of 10 kHz. The optical signal isreceived at a field node (e.g., such as field node 500 a shown in FIG.6), where the optical signal containing the voice information travelsalong a length of optical fiber (e.g., including fiber portion 510 b 1in FIG. 6) to a fiber optical earplug 510 b where such optical signal isagain converted to an electrical signal, and then converted to anacoustic signal at fiber optical earplug 510 b. An exemplary method toachieve this conversion is the reception of the voice-encoded opticalsignal by a photodetector which causes an optical-to-electricalconversion. Part of the electrical energy is rectified, filtered, andused as bias energy for a small electrical circuit that converts theelectrical signal to a baseband (demodulated) acoustic signal outputthrough a miniature loudspeaker within earplug 510 b for audibledetection by a miner or other personnel.

Field node 500 a shown in FIG. 6 also includes a fiber optic microphone510 a which receives an acoustic signal (e.g., the voice of a miner inthe mine). For example, microphone 510 a may be included in a delay coil540 (see FIG. 4) of the Sagnac interferometer at the field node 500 a,may be included in a distinct fiber length in an enclosure 510 (e.g., asmall coil of fiber connected between a sensing cable and a reflector inenclosure 510), or may be included as part of the sensing fiber prior tothe sensing fiber exiting enclosure 510, etc. Regardless of theconfiguration of microphone 510 a, microphone 510 a may be best exposedto the acoustic voice signal when a miner removes a protective cover(not shown) from enclosure 510 to gain access to both fiber opticmicrophone 510 a and fiber optic earplug 510 b. The detected acousticsignal is converted to an optic phase signal by either delay coil 540 orthe small additional coil and then converted to an intensity signal at acoupler, for example, and this optical signal is transmitted to hostnode 100 (e.g., along with other sensed optical information such asdetected beacon signals). At host node 100, the optical signal isconverted to an acoustic signal using host node 100 (e.g., with anintermediate conversion to an electrical signal). The resultant acousticsignal is heard in control room 160 using speakers/headphones 900.

Thus, miners (or other personnel in another application using thistechnique) may communicate with individuals in control room 160, andindividuals in control room 160 may communicate with miners. Further,control room 160 can track miners using beacons 775. Thus, a two-waycommunication and tracking system is provided, with no requirement ofelectrical power in the mine (excluding batteries in beacons 775 worn byeach of the miners).

FIG. 1C illustrates optical detection system 10 of FIG. 1A used in amarine vessel monitoring application. In FIG. 1C, host node 100 andsignal processor 700 are housed in control room 160 or other desirableenvironment (e.g., a remote, stable environment). FIG. 1C illustratesoptical detection system 10 configured to sense disturbances (e.g.,presence of sailors/vessel personnel, acoustic beacons, voices, etc.)aboard vessel 166 (e.g., where vessel 166 includes decks 166 a, 166 b,etc.), where each sensing zone 450, 455, 460 . . . 499, corresponds to agiven area of vessel 166. Details of the interaction of the variouselements in FIG. 1C are omitted for simplicity; however, it isunderstood that the descriptions of like elements in connection withother drawings of the present application are applicable to FIG. 1C.Additional functionality of such elements may also be provided that isuseful in a marine vessel monitoring application. For example, opticaldetection system 10 may be configured such that an alarm condition (orother condition notation such as an updated detection log or display) isprovided when beacon 775 (e.g., worn by a sailor or other marinepersonnel) reaches a predetermined area of vessel 166. Examples of sucha predetermined area may be: one that is off limits to certainpersonnel; a perimeter of vessel 166 which may indicate a man overboard;amongst others. Further, in a marine monitoring application (or otherpersonnel monitoring application) it may be desirable to continuouslymonitor one or more beacons 775 (worn or carried by marine personnel)such that the location of the personnel can be monitored as desired.

FIG. 1D illustrates optical detection system 10 of FIG. 1A used in avehicle monitoring application. That is, it is desired to monitor thelocation of a vehicle 775 a (or personnel within vehicle 775 a) usingbeacon 775 that is carried on or within vehicle 775 a. In FIG. 1D,control room 160 (e.g., including elements of control room 160 such asthose shown in FIGS. 1B and 1C) is within a fence line 167 a of a region167. Vehicle 775 a travels along a roadway 167 b of region 167. In theexample vehicle monitoring system shown in FIG. 1D, optical sensingcables 400 a, 400 b, 400 c, 400 d, 400 e, 400 f, 400 g, and 400 n (andfield nodes 300, 500 a, 500 b, 500 c, 500 d, 500 e, 500 f, 500 n, and600) are secured (or are provided proximate) to fence line 167 a. Thus,optical detection system 10 is configured to sense, for example, thepresence, absence, or location of vehicles within area 167, where eachsensing zone 450, 455, 460, 465, 470, 475, 480, and 499 corresponds to agiven region of area 167. Again, details of the interaction of thevarious elements in FIG. 1D are omitted for simplicity; however, it isunderstood that the descriptions of like elements (e.g., beacons 775 andcontrol room 160) provided in connection with other drawings of thepresent application are also applicable to FIG. 1D. Further, while onevehicle 775 a is shown in FIG. 1D, it is understood that a plurality ofvehicles 775 a may be monitored with optical detection system 10, eachvehicle 775 a having one or more beacon 775 for emitting acousticvibrations at one or more predetermined frequencies unique to eachbeacon 775.

Details of the elements of an exemplary optical detection system 10 (inany of FIGS. 1A-1D) is now described. Referring to FIG. 2, host node 100includes one or more optical sources 110 (e.g., LED sources such assuperluminescent light emitting diodes, edge emitting light emittingdiodes, other light emitting diode sources, lasers, etc.) within anenclosure 112. According to an exemplary embodiment of the presentinvention, optical source 110 may be a broadband optical source operatedin a continuous wave (CW) mode, with an exemplary spectral width beingon the order of 50 nm. Optical source 110 is controlled by a sourcecontrol circuit 111. In the exemplary embodiment now described(described and illustrated in connection with four sensing zones),optical source 110 is connected via an optical cable 120 to a 1×4splitter (such as a 1×4 or 4×4 fiber optic coupler or an integratedoptic splitter) labeled as optical coupler 130. Optical coupler 130divides the light intensity output from optical source 110 into foursignals along respective fibers 140 a, 140 b, 140 c, and 140 d (e.g.,four substantially equal intensity signals) that are each output to arespective input lead of a corresponding optical circulator 150 a, 150b, 150 c, and 150 d (e.g., identical optical circulators 150 a, 150 b,150 c, and 150 d). Output signals are provided along each of fibers 160,161, 162, 163 within fiber optic lead cable 200 from a respective one ofoptical circulators 150 a, 150 b, 150 c, and 150 d.

As provided above, according to certain exemplary embodiments of thepresent invention, linearized Sagnac interferometers are utilized. Aswill be appreciated by one skilled in the art, in order to provide alinearized Sagnac interferometer, the architecture of a traditional loopconfiguration Sagnac interferometer (e.g., typically used to senserotation) is modified (e.g., folded) to allow measurements of phaseperturbations along an optical fiber in a non-looped configuration, forexample, by incorporation of a 1×2 fiber optic coupler. Referring againto FIG. 2 (and FIG. 3), light output from host node 100 travels alongeach of fibers 160, 161, 162, and 163 within lead cable 200 which isconnected to first field node 300. First field node 300 includes anenclosure 310 which houses a series of components.

In FIG. 3, fiber 160 is connected to an input/output lead 315 of anoptical circulator 320. A lead 317 of optical circulator 320 isconnected to a lead 322 of an optical coupler 330 (e.g., a 3×3 fiberoptic coupler 330). A lead 319 of optical circulator 320 is connected toa lead 324 of optical coupler 330.

A lead 332 of optical coupler 330 is connected to a lead 335 of a delaycoil 340. The fiber optic delay coil 340 has a length of, for example,at least twice the length of the zone 450 of an optical fiber 380 inoptical sensing cable 400 a where the midpoint of the sensing loop(e.g., from one output leg of the 3×3 coupler to another) including thesensing optical fiber 380 “unfolded” is within the enclosure 310 formaximum sensitivity. A lead 341 of delay coil 340 is connected to a lead342 of an optical coupler 360 (e.g., a 2×2 fiber optical coupler 360).

A lead 334 of optical coupler 330 is connected to a lead 354 of adepolarizer 350. A lead 326 of optical coupler 330 is tied off and/orthe end crushed to minimize light that is reflected back into opticalcoupler 330. Similarly, a lead 336 of optical coupler 330 is tied offand/or the end crushed to minimize light that is reflected back intooptical coupler 330.

Depolarizer 350 significantly reduces polarization-induced signalfading, allowing inexpensive single mode fiber to be used for all of theoptical components and cable fibers rather than costlypolarization-maintaining fiber. Depolarizer 350 may be one of severalcommercially available depolarizers, such as, for example, arecirculating coupler (single or multiple stage) or a Lyot Depolarizer.A lead 352 of depolarizer 350 is connected to a lead 366 of opticalcoupler 360. A lead 362 of optical coupler 360 is connected to fiber 380in optical sensing cable 400 a. A lead 364 of optical coupler 360 istied off and/or the end crushed to minimize light that is reflected backinto optical coupler 360. Although one example for optical coupler 360is a 2×2 fiber optic coupler, optical coupler 360 is not limited to thatembodiment. For example, a 1×2 fiber optic coupler may be used insteadof a 2×2 fiber optic coupler 360, thereby obviating the tying off ofsecond output lead 364.

Fibers 161, 162, and 163 in lead cable 200 are connected to fibers 370,372, and 374 in field node 300, respectively. These are pass-throughfibers not actively used in first field node 300 or within zone 450, butrather to be used in connection with sensing in other nodes and otherzones. Fibers 370, 372, and 374 are connected to fibers 382, 384, and386 in optical sensing cable 400 a, respectively. Fiber 380 in opticalsensing cable 400 a is used for sensing within zone 450. Fiber 380 inoptical sensing cable 400 a (which had been used for sensing in zone450) is attached to a fiber 580 in intermediate field node 500 a (seeFIG. 4). Fiber 580 is connected to a reflector 581 (e.g., broadbandreflector 581). Disturbances along sensing cable 400 a cause smallchanges in the length of fiber 380. These changes cause non-reciprocalchanges in the phase of the light travelling through the Sagnacinterferometer.

An exemplary operation of first field node 300 shown in FIG. 3 (andpartially in FIG. 4) is now provided. An optical signal (i.e., lightfrom host node 100 entering first field node 300) propagates along fiber160 to lead 315 and enters port 2 of optical circulator 320, and thenexits port 3 of optical circulator 320 through lead 317, and thenpropagates along lead 322 (a length of optical fiber) to optical coupler330. Optical coupler 330 divides the light into optical signals alongtwo counterpropagating paths: a first path of the divided light extendsfrom lead 332 to delay coil 340 along lead 335, and then from lead 341to optical coupler 360 through lead 342; a second path of the dividedlight extends from lead 334 to depolarizer 350 through lead 354, andthen from lead 352 to optical coupler 360 through lead 366. Thus, thelight along the first path is delayed with respect to the light alongthe second path by a time approximately proportional to the length ofdelay coil 340. The two counterpropagating optical signals recombine atoptical coupler 360, and the recombined optical signal exits opticalcoupler 360 along lead 362, and then travels along fiber 380 (forsensing within zone 450) of optical sensing cable 400 a. The recombinedoptical signal enters field node 500 a on fiber 380, and propagatesalong lead 580 to reflector 581, and is then reflected back along fiber380 to first field node 300. This reflected signal is divided into twooptical signals by optical coupler 360, where each of the opticalsignals travels along a counterpropagating path and recombinescoherently at optical coupler 330. The result of the optical signalscounter propagating through the node and zones and recombining atoptical coupler 330 is that the recombined light has an intensity outputproportional to the phase perturbation from the original disturbancealong fiber 380 within optical sensing cable 400 a. This optical signal(having a variable intensity) is output from optical coupler 330 alonglead 324 (i.e., fiber 324) and then along lead 319 into port 1 ofoptical circulator 320. This optical signal propagates from port 1 toport 2 of optical circulator 320, and then along lead 315 to fiber 160of lead cable 200. The signal is transmitted along fiber 160 of leadcable 200 to the interrogator of host node 100.

Referring now to FIG. 4, fibers 384 and 386 in optical sensing cable 400a are connected to fibers 570, 572 in intermediate field node 500 a,respectively. These are pass-through fibers not actively used inintermediate field node 500 a, but rather to be used in connection withsensing in other nodes and zones. Fibers 570, 572 are connected tofibers 584, 586 in optical sensing cable 400 b, respectively. Fiber 582in optical sensing cable 400 b is used for sensing within zone 455.

Fiber 382 from optical sensing cable 400 a is connected to aninput/output lead 515 of an optical circulator 520. The lead 517 ofoptical circulator 520 is connected to a lead 522 of an optical coupler530 (e.g., a 3×3 fiber optic coupler 530). A lead 519 of opticalcirculator 520 is connected to a lead 524 of optical coupler 530.

A lead 532 of optical coupler 530 is connected to lead 535 of a delaycoil 540. The fiber optic delay coil 540 has a length of, for example,at least twice the length of the zone 455 of optical fiber 582 in fiberoptic sensing cable 400 b where the midpoint of the sensing loop (e.g.,from one output leg of the 3×3 coupler to another), including thesensing optical fiber 582 “unfolded” is within enclosure 510 for maximumsensitivity along the zone. A lead 541 of delay coil 540 is connected toa lead 542 of an optical coupler 560 (e.g., a 2×2 fiber optic coupler560).

A lead 534 of optical coupler 530 is connected to a lead 554 of adepolarizer 550. A lead 526 of optical coupler 530 is tied off and/orthe end crushed to minimize light that is reflected back into opticalcoupler 530. Similarly, a lead 536 of optical coupler 530 is tied offand/or the end crushed to minimize light that is reflected back intooptical coupler 530. A lead 552 of depolarizer 550 is connected to alead 566 of optical coupler 560. A lead 562 of optical coupler 560 isconnected to fiber 582 in optical sensing cable 400 b. A lead 564 ofoptical coupler 560 is tied off and/or the end crushed to minimize lightthat is reflected back into optical coupler 560. Although an exemplaryoptical coupler 560 is a 2×2 fiber optic coupler, the optical coupler560 is not limited to that embodiment. For example, a 1×2 fiber opticcoupler may be used instead of a 2×2 fiber optic coupler 560, therebyobviating the tying off of lead 564.

An exemplary operation of field node 500 a shown in FIG. 4 is nowprovided. An optical signal (i.e., light from host node 100 enteringfield node 500 a) propagates along fiber 382 to lead 515 and enters port2 of optical circulator 520, and then exits port 3 of optical circulator520 through lead 517, and then propagates along lead 522 (a length ofoptical fiber) to optical coupler 530. Optical coupler 530 divides thelight into optical signals along two counterpropagating paths: a firstpath of the divided light extends from lead 532 to delay coil 540 alonglead 535, and then from lead 541 to optical coupler 560 through lead542; a second path of the divided light extends from lead 534 todepolarizer 550 through lead 554, and then from lead 552 to opticalcoupler 560 through lead 566. Thus, the light along the first path isdelayed with respect to the light along the second path by a timeapproximately proportional to the length of delay coil 540. The twocounterpropagating optical signals recombine at optical coupler 560, andthe recombined optical signal exits optical coupler 560 along lead 562,and then travels along fiber 582 (for sensing within zone 455) ofoptical sensing cable 400 b. The recombined optical signal enters fieldnode 500 b (see FIGS. 1A-1D) on fiber 582, and is reflected back (usinga reflector in field node 500 b similar to reflector 581 in field node500 a) along fiber 582 to field node 500 a. This reflected signal isdivided into two optical signals by optical coupler 560, where each ofthe optical signals travels along a counterpropagating path andrecombines coherently at optical coupler 530. The result of the opticalsignals counter propagating through the node and zones and recombiningat optical coupler 530 is that the recombined light has an intensityoutput proportional to the phase perturbation from the originaldisturbance along fiber 582 within optical sensing cable 400 b. Thisoptical signal (having a variable intensity) is output from opticalcoupler 530 along lead 524 (i.e., fiber 524) and then along lead 519into port 1 of optical circulator 520. This optical signal propagatesfrom port 1 to port 2 of optical circulator 520, and then along lead 515to fiber 382 (and pass through fiber 370) to fiber 161 of lead cable200. The signal is transmitted along fiber 161 of lead cable 200 to theinterrogator of host node 100.

The pattern of field nodes 500 a, 500 b, etc. and optical sensing cables400 a, 400 b, etc. is repeated, as desired, and utilizing the number ofavailable optical fibers within the cable. Other system level topologies(e.g., branching, bi-directional/redundancy, etc.) are contemplatedusing this modular approach. Each optical sensing cable 400 a, 400 b,etc. may be used to provide an acoustically independent sensing zone.FIG. 5 illustrates final field node 600 including an enclosure 610 forreceiving final optical sensing cable 400 n. Optical sensing cable 400 nincludes a fiber 680 which is connected to a reflector 681 (e.g.,broadband reflector 681).

Referring back to FIG. 2, optical intensity signals proportional to thephase perturbations within each zone (e.g., due to mechanical oracoustic vibrations sensed) are returned to host node 100 (which may beconsidered an interrogator) by way of fibers 160, 161, 162, and 163 andthen through circulators 150 a, 150 b, 150 c, and 150 d after conversionfrom a phase signal to an intensity signal at coupler 330 or 530, etc.Circulators 150 a, 150 b, 150 c, and 150 d are configured to behave insuch as way as to allow signals from fiber 160 to pass through to afiber 174, for signals from fiber 161 to pass through to a fiber 173,for signals from fiber 162 to pass through to a fiber 172, and forsignals from fiber 163 to pass through to a fiber 171. However, thecirculators 150 a, 150 b, 150 c, and 150 d prevent light from passingfrom: fiber 160 or fiber 174 to fiber 140 a; fiber 161 or fiber 173 tofiber 140 b; fiber 162 or fiber 172 to fiber 140 c; and fiber 163 orfiber 171 to fiber 140 d, etc. Light from fiber 174 is converted to anelectrical current signal at a photodetector 175. Likewise, light fromfiber 173 is converted to an electrical current signal at aphotodetector 176, light from fiber 172 is converted to an electricalcurrent signal at a photodetector 177, and light from fiber 171 isconverted to an electrical signal at a photodetector 178. The electricalsignals converted by photodetectors 175, 176, 177, and 178 may be verylow noise signals, and the photodetectors 175, 176, 177 and 178 may havedark current less than about 0.5 nA.

The outputs of photodetectors 175, 176, 177, and 178 are then amplifiedusing transimpedance amplifiers 180 (e.g., amplifiers of very lowdistortion (less than −40 dB), high gain bandwidth (on the order of500-2,000 MHz), and noise less than 1 nV/√Hz (such as the model AD8099,produced by Analog Devices, Inc.)). Multiple stages of furtheramplification may follow each transimpedance amplifier 180 as is knownby those skilled in the state of the art. The electrical outputs ofamplifiers 180 are filtered using filters 181. Use of high qualityphotodetectors, amplifiers, and filters desirably produces signals withfidelity sufficient for advanced signal processing desired for robustclassification of detected events and alarm generation (or otherindications based on mechanical/acoustic vibration) without falsealarms. The signals output from filters 181 are sampled by A/Dconverters (ADCs) 182. The sampled electrical signals from ADCs 182 arereceived by one or more Field Programmable Gate Arrays (FPGAs) 184.

FPGAs 184 may be configured to perform high speed signal pre-processing.Such FPGAs 184 are typically used to perform filtering and Fast FourierTransforms (FFTs) of the sampled data from each zone to determine theinstantaneous spectrum of the disturbance(s) along each zone. Furtherprocessing is performed by a microprocessor 186 as shown in FIG. 2.Communication with outside security system processors and otherperipheral devices is accomplished with an interface chip 188. Interfacechip 188 may be for example, an RS-232 interface chip or a USBtransceiver.

An exemplary signal processing sequence is accomplished as follows. Fromeach sensing zone (e.g., zone 450, zone 455, zone 460, etc.), ADCs 182digitize a set of data samples (e.g., at an exemplary rate of 8192samples per second). In such an example, FPGA 184 performs a 8192 sampleFFT to produce spectra, which are output to the microprocessor 186.Microprocessor 186 groups the spectra output from FPGA 184 into datawindows (e.g., on the order of 0.25 seconds).

In such an example, a series of spectral masks are created by processingsignals generated during the introduction of known events (where suchevents may be configured depending upon the application). In a minemonitoring application such an event may be a characteristic of anindividual miner's beacon, a characteristic of an individual miner'svoice in a mine, etc. Spectra generated by FPGA 184 during these eventsare saved, for example, in a database, a look-up table, or other datastorage techniques. Each of these spectral masks is further modified tocreate a dynamic signal threshold. The spectrum of the received datawithin each data window is compared to the signal thresholds. Apersistence requirement is established that requires “m” spectra toexceed a spectral mask for every “n” contiguous time windows which, whentrue, is reported as an alarm condition or as the detection (existence)of a particular beacon in or near a specific zone. The use ofpersistence helps minimize false alarms due to instantaneous (non-alarm)events of high energy.

The dynamic threshold is continually updated by multiplying auser-defined coefficient to a single value is calculated for eachfrequency band within a spectrum by summing the values of a commonfrequency band from all of the zones in an environmental zone (where theenvironmental zone is a set of real sensing zones artificially groupedby the user). These values are integrated over a user-defined time span.This dynamic threshold is used to compensate for non-instantaneousenvironmental effects impacting multiple zones (e.g., lasting on theorder of seconds to hours), such as rain, hail, highway traffic, trains,etc. The shorter this time span of the dynamic threshold integration,the more rapidly the dynamic threshold changes. The longer this timespan, the more the dynamic threshold response is damped. In addition,the amount that any one instantaneous spectrum can bias the dynamicthreshold can also be limited to prevent single events (such as animpact from a falling tree branch) from having an undue impact upon thethreshold.

Electrical outputs from ADCs 182 in host node 100 may be combined anddistinguished by use of a multiplexer, switch, or other appropriatemechanism 1000 to an amplifier or line driver 1011 to provide an audiooutput of any zone desired by a user. Providing an audible outputenhances the functionality of optical detection system 10 by enablingthe user to hear the detected events as alarms are generated.

FIG. 6 is an external illustration of intermediate field node 500 a (asopposed to the internal block diagram view shown in FIG. 4). Field node510 a includes enclosure 510, fiber optic microphone 510 a, and fiberoptic earplug 510 b (where the function of microphone 510 a and earplug510 b have been described above). Earplug 510 b is connected to theinternal portion of enclosure 510 using optical fiber 510 b 1. As fieldnode 510 a relates to node “n+1,” it is understood that the cable forsensor “n” terminates at enclosure 510 (using a reflector or the like),and that the cable for sensor “n+1” exits from enclosure 510. As will beappreciated by those skilled in the art, any node (a first field node,an intermediate field node, a final field node, etc) may include thefunctions described in connection with FIG. 6 such as that of microphone510 a and earplug 510 b.

FIG. 7 is a cross sectional view of an optical sensing cable 750including four fibers or strands (such as optical sensing cable 400 ashown in FIGS. 3-4). Although a number of different sensing cableconfigurations are possible, optical sensing cable 750 shown in FIG. 7includes an outer jacket 752 and one or more strength members 754.Strength member 754 provides longitudinal tensile strength to sensingcable 750 as well as some bend-limiting for the benefit of thereliability of elements contained within sensing cable 750. Outer jacket752 may be made from, for example, one of a series of commonelastomer/rubber materials such as polyurethane, polyethylene, butyl, ornitrile. Cable 750 may include sub units 756. Each sub unit 756 containsa jacket 756 a (e.g., a PVC jacket), a strength layer 760 (e.g., formedfrom a material such as a stainless steel, polyimide fiber like theKevlar® polyimide fiber marketed by E.I. duPont de Nemours & Co., Inc.of Wilmington, Del.), and an optical fiber 758. Optical fiber 758 mayinclude a jacket (not shown) formed from a material such as Hytrel®thermoplastic polyester elastomers marketed by E.I. duPont, nylon, orsilicone.

The optical detection systems 10 shown in FIGS. 1A-1D relate to alinearized Sagnac type of architecture; however, the present inventionis not limited to such an architecture. One example of an alternativearchitecture is a Time Division Multiplexing (TDM) system opticalarchitecture such as that shown in FIG. 8. In this embodiment, apersonnel safety system 1010 is configured as an infinite impulseresponse interferometer array. The functions of various of the elementsdescribed in connection with FIG. 8 are similar to those described abovein connection with FIGS. 1A-1D. A control room 1160 includes controlroom electronics 1160 a. Control room electronics 1160 a includes a hostnode 1100 (including an interrogator), a signal processor 1700, amicrophone 1800, and speakers/headphones 1900. Host node 1100 isconnected to an optical sensing cable 1400 using a lead cable 1200,where lead cable 1200 extends into a mine 1165 (below the ground level1165 a and the above mine floor 1165 b).

Optical sensing cable 1400 contains a series of interferometers (e.g.,Fabry-Perot interferometers) that are each a segment of an optical fiberof optical sensing cable 1400. The interferometers are bounded by a pairof Fiber Bragg Gratings (FBGs). More specifically, segment 1400 a isbounded by FBGs 1410 a, 1410 b. Likewise, segment 1400 b is bounded byFBGs 1410 b, 1410 c. Likewise, segment 1400 c is bounded by FBGs 1410 c,1410 d, and so on, until the final segment terminates at FBG 1410 n.According to an exemplary embodiment of the present invention, each ofthe FBGs (e.g., 1410 a, 1410 b, 1410 c, 1410 d, 1410 n) are periodicperturbations to the crystallographic structure of the fiber. Suchperturbations may be created by an interference pattern using a laserbeam as is well known by those skilled in the art. Exemplary ones of theFBGs have a peak reflection on the order of one percent, and have aspectral width (full width at half maximum or FWHM) of typically 4-6 nm.The center wavelength of exemplary FBGs is dependent upon the type ofmultiplexing used within the system. The purposes of the interrogator(within host node 1100) are to illuminate the array of interferometers(e.g., with very narrow line width light, for example, on the order of0.1-10 kHz FWHM) and to provide an electrical output which isproportional to the acoustic input to each interferometer. An example ofsuch an interrogator, which includes the optical source, is a low phasenoise laser such as an external cavity laser or a fiber laser. A phasesignal is imposed upon the light, which is also pulsed, with pulsewidths equal to twice the distance between adjacent FBGs. The pulses aretransmitted to the linear sensor array including the interferometers.The interferometers (e.g., the fiber segments bound by a pair of FBGgratings) sense acoustic and/or mechanic vibrations (e.g., an emissionfrom a beacon 1775), and after return from the linear sensor array tohost node 1100, the optical signals (having been perturbed by phasechanges caused by vibrations, etc.) are demodulated (e.g., downconverted) and available for post processing (e.g., spectral analysis,mask comparison, etc.) by processor 1700 (e.g., a microprocessor, a PC,etc.) where such vibration is processed to interpret the event (e.g., tounderstand the location of the miner, etc. by detecting unique beaconoutputs).

In certain exemplary embodiments of the present invention, a separatefiber within the optical sensing cable carries the light/optical signalthat contains the voice information (e.g., voice information from thecontrol room to an earplug at a field node, etc.). FIG. 8 alsoillustrates a field node 1500 which may be similar to node 500 a shownin FIG. 6. Final field node 1500, which is illustrated at the end ofcable 1400, may be placed in any position in mine 1165 as desired, and,for example, a plurality of field nodes may be provided along cable1400. Because final field node 1500 may include a fiber optic microphoneand earplug (as shown in FIG. 6), and because control room electronics1160 a include microphone 1800 and speakers/headphones 1900, thebi-directional communication (without need for local electrical power)described above with respect to optical detection system 10 of FIGS.1A-1D is also provided by personnel safety system 1010 of FIG. 8. Use ofa TDM system architecture is not limited to mine safety applicationssuch as is shown in FIG. 8. Rather, such an architecture may be used inany of a number of personnel safety and/or monitoring applications suchas marine safety (FIG. 1C), and vehicle monitoring (FIG. 1D), amongstothers.

As will be appreciated by those skilled in the art of mine safety, it isalso desirable to provide miners with passive ways to move toward a mineexit in the event of a power failure (where there is no light toindicate the direction of the mine exit). In some applications, coneshave been strung along a line of a wall of the mine, thereby creating alifeline. When a miner runs a hand along the line, the miner feels thecones. When the cones extend from small end to large end, the minerknows that he or she is heading toward the mine exit. In contrast, whenthe cones extend from large end to small end, the miner knows that he orshe is not heading toward the mine exit. In accordance with the presentinvention, a fiber cable (e.g., cable 1400 shown in FIG. 8, segments 400a, 400 b, . . . 400 n shown in FIG. 1B, etc.) may be used to performsuch a directional function in addition to its function of transmittingoptical information. FIG. 9 illustrates a cable 950 including aplurality of cone shaped structures 952 disposed thereon. As describedabove, a miner may feel cable 950 as he or she heads in a givendirection to be certain that he or she is heading toward the mine exit.

The present invention also includes methods of operating opticaldetection systems such as the optical detection systems illustrated anddescribed in connection with FIGS. 1A-1D and FIGS. 2-8 (in connectionwith personnel safety applications). FIG. 10 illustrates an example ofsuch a method implemented in a closed-loop fashion. At step 1000, aplurality of predetermined characteristics of events (e.g.,characteristics of individual miner's beacons such as a temporalcharacteristic an example of which is a pulse train carrier as part of acode division multiple access methodology, characteristics of miner'svoices in a mine, etc.) to be monitored using an optical detectionsystem are stored in memory. By “predetermined” is meant determinedbeforehand, so that the predetermined characteristic must be determined,i.e., chosen or at least known, in advance of some event such asimplementation of the method. Depending upon the application of theoptical detection system, such events (and therefore, the predeterminedcharacteristics of such events) may vary broadly. For example, in a minesafety application, exemplary events may include miner's voices in amine, beacon transmissions of a miner, etc. In a marine personnelmonitoring application, such events may include sailor's voices in anarea of a ship (e.g., a compartment of a large vessel), beacontransmissions of a sailor, etc. Further still, the characteristics ofthe events provided may vary broadly. As provided above, such acharacteristic may be a spectra or a spectrum of a known event. Such aspectrum may be an energy profile over a plurality of frequencies, etc.

In one specific example, in order to provide the characteristics at step1000, a number of sub steps are completed. In a first substep, awindowing function (such as a Hanning function or Beckman function) isapplied to a sampled set of data points within a series of time windowsduring a series of known events (e.g., beacon transmissions, talking,etc.). In a second substep, a spectrum is created by applying a FastFourier Transform (FFT) on the windowed data. In a third substep, thespectrum is scaled in a way to include a population of system responsesto a series of similar events (e.g., in such a way as to minimize falsealarms) to create a spectral mask. In a fourth substep, the resultantspectral mask is associated with each event and is stored in a datastructure (e.g., a database or other similarly retrievable structure).

At step 1002, a detected characteristic obtained from the opticaldetection system during normal operation (e.g., obtained from the hostnode by processing of optical intensity information received from thevarious field nodes and sensing zones) is compared to the plurality ofpredetermined characteristics stored in memory. Referring again to thespectra example described above, step 1002 may include two substeps. Ina first substep, windowed samples of data are acquired during normaloperation, and spectra of this data are generated as a function of time.Then, in a second substep, the spectra generated during normal operationare compared to those previously associated with alarm events and stored(e.g., compared to the characteristic provided in step 1000).

At step 1004, a determination is made as to whether there is anacceptable level of matching between the detected characteristic fromstep 1002 and at least one of the plurality of predeterminedcharacteristics stored in memory in step 1000. If there is no suchacceptable level of matching (i.e., a “No” answer at step 1004), thenthe process returns to step 1002 and further comparisons are made withupdated data. If there is such an acceptable level of matching (i.e., a“Yes” answer at step 1004) then an alarm (or other notation such as anupdated detection log or display) may be generated at step 1008.

As will be appreciated by those skilled in the art, certain types ofevents may be of a momentary nature, and a momentary match (i.e., amomentary acceptable level of matching at step 1004) may suffice togenerate an alarm at step 1008. However, other types of events may be ofsuch a type where it is appropriate to confirm that the event continuesfor a predetermined period of time. In such a case, even if there issuch an acceptable level of matching (i.e., a “Yes” answer at step1004), the process may not immediately generate an alarm, but may ratherproceed to step 1006 where a determination is made as to whether theacceptable level of matching is present for a predetermined period oftime (e.g., or apply a persistence test to the processed operationaldata to see if it exceeds an alarm threshold, where such threshold maybe the predetermined period of time, or some other threshold). If theanswer at step 1006 is “Yes,” then an alarm is generated at step 1008.If the answer at step 1006 is “No,” then the process proceeds to step1002 for continued monitoring. The step 1006 of determining if theacceptable level of matching is present for a predetermined period oftime can be accomplished in a closed loop fashion wherein a counter isupdated for each incremental time period during which there is anacceptable level of matching.

FIG. 11 illustrates another method of operating an optical detectionsystem in accordance with an exemplary embodiment of the presentinvention, in connection with personnel safety applications. At step1100, a plurality of predetermined frequency pairs (e.g., frequencypairs associated with personnel beacons) are stored in memory. At step1102, acoustic data (e.g., a detected spectrum associated with the data)are continuously sampled and compared with the set of predeterminedfrequency pairs stored in memory. At step 1104, a determination is madeas to whether there is a match between the detected spectrum and thefrequency pairs stored in memory. If the answer at step 1104 is “No,”the process returns to step 1102 for additional sampling. If the answerat step 1104 is “Yes,” the method proceeds to step 1106 where thepersonnel member's name/identifier (e.g., the miner's name/identifier,etc.) corresponding to the detected frequency pairs is displayed withinthe sensing zone in which the detection was made.

At step 1108 a determination is made as to whether a particular expectedfrequency pair (e.g., a miner's beacon signal) has not been detectedanywhere within a predetermined period (e.g., a time period after whicha miner/personnel member is considered lost). If the answer at step 1108is “Yes,” the process proceeds to step 1110 where an alarm is generated(e.g., a “Miner Lost” alarm, a “Sailor Lost” alarm, or other “PersonnelMember Lost” alarm, etc.). If the answer at step 1108 is “No,” theprocess returns to step 1102 for additional sampling. It should be notedthat beacons (e.g., beacons 775/1775), and hence steps 1102/1104 in FIG.11, may operate the frequencies in tandem rather than simultaneously.

Although the present invention has largely been described in connectionwith monitoring of miners (e.g., a miner safety application), it is notlimited to such embodiments. For example, the personnel safety systemmay be used in connection with any of a number of personnel monitoringapplications.

The optical fibers and cables illustrated and described herein may bearranged in any desired configuration. For example, each of the fibersmay be provided in a single length between elements, or in multiplelengths, as desired. In a specific example, fiber 160 in FIG. 3 connectsto port 2 of optical circulator 320 through lead 315; however, it isunderstood that lead 315 may be part of fiber 160 if desired. Likewise,port 3 of optical circulator 320 and optical coupler 330 are connectedthrough leads 317 and 322; however, it is understood that leads 317 and322 may be part of the same length of optical fiber if desired.

Although the present invention has primarily been described inconnection with lengths of optical sensing cable 400 a, 400 b, etc.sensing disturbances (e.g., as in FIGS. 1A-1D), the present invention isnot limited to such embodiments. For example, one or more point sensingtransducers may be integrated into each of the sensing zones. Such pointsensing transducers may be used to sense a disturbance at a specific“point” along a sensing cable segment as opposed to general sensinganywhere along the sensing cable segment. Further, such point sensingtransducers may include elements or structure distinct from (and inaddition to) the sensing cable segment.

Although the present invention has been described in connection withcertain exemplary elements (e.g., the elements illustrated and describedin connection with FIGS. 2-5), it is not limited to those elements. Theoptical detection system may use any of a number of types of componentswithin the scope and spirit of the claims.

Although illustrated and described above with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

What is claimed:
 1. A method of operating a personnel monitoring systemcomprising: (a) storing a plurality of predetermined characteristics ofevents to be monitored using an optical detection system in memory; (b)comparing a detected characteristic obtained from the optical detectionsystem to the plurality of predetermined characteristics stored inmemory; and (c) determining if there is an acceptable level of matchingbetween the detected characteristic and at least one of the plurality ofpredetermined characteristics stored in memory.
 2. The method of claim 1further comprising the step of (d) generating an alarm condition if itis determined that there is an acceptable level of matching at step (c).3. The method of claim 1 wherein, if it is determined that there is anacceptable level of matching at step (c), the method further comprisesthe step of (d) determining if the acceptable level of matching ispresent for a predetermined period of time in a closed loopconfiguration wherein a counter is updated for each incremental timeperiod during which there is an acceptable level of matching.
 4. Themethod of claim 1 wherein, if it is determined that there is anacceptable level of matching at step (c), the method further comprisesthe step of (d) determining if the acceptable level of matching ispresent for a predetermined period of time.
 5. The method of claim 4further comprising the step of (e) generating an alarm condition if itis determined that the acceptable level of matching is present for apredetermined period of time in step (d).
 6. The method of claim 5wherein step (a) includes: (a1) applying a windowing function to asampled set of data points within a series of time windows during aseries of known events; (a2) creating a spectrum by applying a FastFourier Transform (FFT) to the set of data points to which the windowingfunction has been applied; (a3) scaling the spectrum to include apopulation of system responses to a series of similar events to create aspectral mask; (a4) associating the spectral mask with each of theseries of known events; and (a5) storing the associated spectral mask ina data structure accessible by the optical detection system.
 7. Themethod of claim 6 wherein step (b) includes: (b1) acquiring windowedsamples of data during operation of the optical detection system; (b2)generating spectra of the windowed samples of data as a function oftime; and (b3) comparing the spectra generated in step (b2) to thespectral mask stored in step (a5) of step (a).